It is known that very hot sand can be cooled to about 100.degree. C. by spraying it with water. As for cooling below 100.degree. C., this is conventionally done without spraying water in order to obtain very dry sand.
For example, to cool foundry sand from 100.degree. C. to 40.degree. C. and recover the heat, several known methods can be used in which the cooling fluid is either:
atmospheric air in direct contact with the sand; or
water in indirect contact via the walls of a heat exchanger.
In both these cases, the heat transfer coefficients are fairly low, typically a few tens of kcal/h.m.sup.2..degree. C., whereas the heat transfer coefficients by direct evaporation and direct condensation of water are about 1000 kcal/h.m.sup.2..degree. C.
It would therefore be advantageous to use water as the cooling fluid. However, when the sand to be cooled is sand which has been dried in a rotating furnace, it is pointless to cool it to 40.degree. C. by a process which wets it. The cooled sand must be perfectly dry for use in a foundry mould or for transport. Further, it is often useful to recover heat from the sand as it is cooled.
Preferred embodiments of the present invention provide apparatus for extracting heat from a water-absorbant powdered substance at temperatures below 100.degree. C. The apparatus makes it possible to obtain cooled dry substance and to recover the heat extracted.